[GRASS-user] Interpreting r.watershed accumulation map

[Laurent C.]

Hello Rich,

According to the r.watershed manual:
"Output accumulation map contains the absolute value of each cell in
this output map is the amount of overland flow that traverses the
cell. This value will be the number of upland cells plus one if no
overland flow map is given. If the overland flow map is given, the
value will be in overland flow units. Negative numbers indicate that
those cells possibly have surface runoff from outside of the current
geographic region. Thus, any cells with negative values cannot have
their surface runoff and sedimentation yields calculated accurately.
"

So basically the darker the cell, the larger the drained area. Darker
cells therefore show the flow courses according to the topography.

Note that Itzï does not need an output to run. If you set a simulation
with only the DEM, an uniform friction coefficient and an uniform
rainfall, the water will simply accumulate in the depressions, giving
you an insight of the flow patterns.

Regards,
Laurent


2016-10-10 11:23 GMT-05:00 Rich Shepard <rshepard at appl-ecosys.com>:
>   The input to r.watershed (be_cropped) has elevation range of 148.2m -
> 168.4m. The accumulation output map has values ranging from
> -89397.3-38013.4. An image of a portion of the map is attached.
>
>   How do I interpret the colors on the map since the legend shows only a
> narrow blue bar on a black background? As there are cells with high negative
> values does this map contain any useful information?
>
>   Given such flat land (the area in the image has an elevation difference of
> approximately 1m) I have not identified a module that produces insights into
> surface runoff patterns. I've tried several with no success; r.lake does
> produce water depths based on the elevation of a blockage.
>
>   Itzi does not work on these data because there are no stream channels for
> use by r.water.outlet.
>
>   Have I missed something?
>
> Rich
> _______________________________________________
> grass-user mailing list
> grass-user at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/grass-user